ARTICLES PUBLISHED ONLINE: 19 APRIL 2009 | DOI: 10.1038/NMAT2430 Spraying asymmetry into functional membranes layer-by-layer Kevin C. Krogman 1,2 , Joseph L. Lowery 1,2 , Nicole S. Zacharia 2,3 , Gregory C. Rutledge 1,2 and Paula T. Hammond 1,2 * As engineers strive to mimic the form and function of naturally occurring materials with synthetic alternatives, the challenges and costs of processing often limit creative innovation. Here we describe a powerful yet economical technique for developing multiple coatings of different morphologies and functions within a single textile membrane, enabling scientists to engineer the properties of a material from the nanoscopic level in commercially viable quantities. By simply varying the flow rate of charged species passing through an electrospun material during spray-assisted layer-by-layer deposition, individual fibres within the matrix can be conformally functionalized for ultrahigh-surface-area catalysis, or bridged to form a networked sublayer with complimentary properties. Exemplified here by the creation of selectively reactive gas purification membranes, the myriad applications of this technology also include self-cleaning fabrics, water purification and protein functionalization of scaffolds for tissue engineering. N aturally occurring membranes, such as those found in plants, cell walls and organs including the epidermis and intestinal wall, derive their ability to segregate two different environments largely from the asymmetry established by their protein constituents 1,2 . Instead of functioning as a uniform barrier, the cross-section of these membranes varies according to their purpose, enabling interior and exterior portions of the membrane to serve very different roles. Segregation of structure is also observed in porous polymer membranes, where a thin effective separation layer is formed at the upstream surface of the membrane, while the bulk material remains porous and less densely packed 3 . Although identical in chemical composition, asymmetric arrangement of two morphologies provides the membrane with mechanical robustness whereas separation is regulated predominantly by the thin barrier layer of material near the surface. Aside from mechanical integrity, however, the phase inversion technique used to generate asymmetric polymer membranes does not introduce functional activity to the bulk matrix, thereby underutilizing the full potential of the substrate material. Here we present a novel process capable of two distinct flow-rate-dependent modes of electrostatic deposition by which multiple functionalities can be introduced into a single engineered textile. Similar to the way in which many naturally occurring membranes simultaneously regulate mass transfer and undergo chemical reactions with solute molecules, this technique enables portions of the textile to act as an inert barrier while the bulk material acts as a high-surface-area scaffold capable of a wide variety of functionalities. The layer-by-layer (LbL) assembly technique enables the deposition of ultrathin uniform films by the sequential electrostatic deposition of charged polymers 4–6 , nanoparticles 7–10 , biological templates 11 or biologically active species 12 . An inherently charged substrate is serially exposed to solutions of oppositely charged species, which adsorb to the developing film at rates that enable nanometre-scale control of the film thickness 13 . In recent years, an 1 Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA, 2 Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA, 3 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. *e-mail:hammond@mit.edu. extension of this technique has been developed by which solutions of charged species are sprayed onto the desired substrate 14–16 . Similar to the traditional dipping process, assembly occurs through electrostatic interactions between areas of local charge density on oppositely charged species, but process times can be reduced more than 25-fold by convectively transporting charged species to the surface. Planar non-porous substrates such as silicon and glass are readily coated by either technique and, when exposed to similar solutions, show ostensibly similar growth rates and final film properties 14,16 . Electrospun (ES) fibres enable the generation of porous polymer scaffolds, which can be tuned for fibre size and surface area 17 and chemically modified using a number of methods 18–20 . By drawing a pressure gradient across porous substrates during the spray-assisted layer-by-layer (spray-LbL) process, we have found that highly conformal coatings can be developed on individual fibres, wires or pores throughout the thickness of the bulk porous substrate. This process retains the flexibility, speed and ambient processing conditions that make spray-LbL an attractive deposition technique, and is capable of creating exceptionally high-surface-area coatings; applications of relevance include self-cleaning photocatalysis 19,21,22 , conformal surface passivation 23–26 for corrosion protection, or biocatalytic membranes for pharmaceutical or biofuel applications. To demonstrate the conformal coating of individual fibres within a material, parallel-plate electrospinning was used to create flexible non-woven mats of microscale nylon 6,6 fibres (average fibre diameter D = 1.64 ± 0.25 μm) from hexafluoroisopropanol solutions (Fig. 1a,b) 27 . Selecting poly(dimethyldiallylammonium chloride) (PDAC) as the cationic species and amphoteric titanium dioxide nanoparticles (which have been synthesized at a pH above the isoelectric point) as the anionic species, a sprayed deposition can be performed. Chosen for its photocatalytic capabilities, this system presents an ideal candidate for catalysis applications by implementing a surface coating on a high-surface-area scaffold. 512 NATURE MATERIALS | VOL 8 | JUNE 2009 | www.nature.com/naturematerials © 2009 Macmillan Publishers Limited. All rights reserved.